Certain chronic diseases destroy the functional cells in affected organs. Mammals with such diseases are often unable to produce proteins or hormones necessary to maintain homeostasis and usually require numerous exogenous substances to survive. Transplanting healthy organs or cells into a mammal suffering from such a disease may be necessary to save the mammal's life. This type of therapy is generally regarded as a last alternative to curing an otherwise fatal condition. Such transplants, however, are often rejected by the body due to an immune response initiated in response to the foreign tissue or cells. Presently, the only recourse to combat this immune response is to administer chronic nonspecific immunosuppression agents. Unfortunately, this only trades the complications of one chronic disease with other complications caused by the immunosuppression agent.
One disease which scientists have attempted to treat with organ and/or cellular transplants but have had very limited success is diabetes mellitus. Diabetes mellitus is a prevalent degenerative disease in mammals. It is characterized by a relative or complete lack of insulin secretion by the beta cells within the islets of Langerhans of the pancreas or by defective insulin receptors.
This insulin deficiency prevents normal regulation of blood glucose levels and often leads to hyperglycemia and ketoacidosis. When administered to a mammal, insulin promotes glucose utilization, protein synthesis, formation and storage of neutral lipids and the growth of certain cell types.
In the United States alone there are approximately 13 million diabetics. Of these, 2.6 million are insulin dependent diabetics. Drug & Market Dev., 4:210 (1994). Health care analysts estimate that diabetes costs $92 billion a year resulting from medical costs and lost productivity.
The various forms of diabetes have been organized into a series of categories developed by the National Diabetes Data Group of the National Institutes of Health. Type I diabetes in this classification scheme includes patients dependent upon insulin to prevent ketosis. This group of diabetics was previously called juvenile-onset diabetes, brittle diabetes or ketosis-prone diabetes. Type I diabetes is caused by an autoimmune reaction that causes complete destruction of beta cells.
Type II diabetes is classified as adult-onset diabetics. The diabetic patient may or may not be insulin dependent. Type II diabetes can be caused by a number of factors. For most mammals with Type II diabetes, the beta islet cells are defective in the secretion of insulin.
There are many therapies currently used to treat diabetes, however, each has its limitations. The major problem confronting most patients with diabetes mellitus is that currently available therapies fail to prevent the complications of the disease process. The most common method of treating Type I diabetes in mammals is providing an endogenous source of insulin such as porcine, bovine or human insulin. Insulin injection therapy prevents severe hyperglycemia and ketoacidosis, but does not completely normalize blood glucose levels. This treatment further fails to prevent the complications of the disease process, including premature vascular deterioration. Premature vascular deterioration is the leading cause of morbidity among diabetic patients. Furthermore, complications resulting from long-term diabetes include renal failure, retinal deterioration, angina pectoris, arteriosclerosis, myocardial infarction and peripheral neuropathy.
A second method of treating diabetes is by transplanting the pancreas in conjunction with the administration of chronic nonspecific immunosuppression agents. This treatment is usually given to an individual who has advanced diabetes, such as an individual with kidney failure. Whole pancreas transplantation can be successfully done with a 75% one year survival rate, but surgical transplantation of the pancreas is very difficult. Furthermore, since the entire organ must be donated, the only practicable source is a deceased donor. In addition, when cyclosporine, the most common immunosuppressive drug used for organ transplants, is administered in a dosage necessary to suppress the immune response, the drug inhibits pancreatic cell function. Furthermore, the steroids that are often administered with an organ transplant often cause the patient to become diabetic.
A third treatment involves transplanting islet of Langerhans cells into the diabetic patient. However, islet transplantation has been generally unsuccessful due to the aggressive immune rejection of islet grafts. (Gray, 1991, Immunology Letters 29:153; Jung et al., 1990, Seminars in Surgical Oncology 6:122). In particular, successful transplantation of isolated pancreatic islet cells has been very difficult to achieve due to the chronic administration of immunosuppressive drugs required to prevent organ rejection of the cells following transplantation. These dosages of immunosuppressive drugs can cause increased susceptibility to infection, hypertension, renal failure and tumor growth. Furthermore, unlike most organ transplants, islet cells must grow their own blood supply following implantation in the host in order for the cells to survive. Conventional transplantation techniques do not provide the necessary factors to stimulate the production of new blood vessels.
Thus, to successfully transplant cells in a mammal, it is necessary that the cellular transplants are not rejected by the recipient and have the capacity to grow upon transplantation. As a commercial reality, it is further necessary that a sufficient quantity of cells are available for transplantation. Traditionally, the number of cellular transplants have been limited by the inability to adequately collect and store a sufficient number of cells for transplantation. Conventional storage techniques, such as cryopreservation, often damage a large quantity of the stored cells. Porcine islet cells, for example, are extremely fragile and easily dissociate into fragments or single cells upon thawing.
The present invention alleviates many of the problems associated with the current therapies for chronic diseases that destroy the functional cells of vital organs. Specifically, the present invention provides a method of creating systemic tolerance to subsequent transplants in the mammal. Furthermore, the present invention solves the problems associated with the conventional therapies for diabetes mellitus, by providing a method of transplanting pancreatic islets cells into a diabetic mammal, whereby the cellular transplants produce insulin in the diabetic mammal. The present inventor has previously demonstrated extended functional survival of islet cells allografts and xenografts in the testis. (Selawry et al., 1989, Diabetes 38:220.) It has been surprisingly discovered in accordance with the present invention that an immunologically privileged site can be created in a mammal by transplanting Sertoli cells to a nontesticular site in a mammal. The newly created immunologically privileged site allows the transplantation and survival of cells that produce biological factors useful in the treatment of diseases, especially diabetes. In addition to creating an immunologically privileged site, the Sertoli cells produce cell stimulatory factors which enhance the maturation, proliferation and functional capacity of cells. Sertoli cells have further been found to enhance the recovery rate and viability of mammalian cells stored by techniques such as cryopreservation.